The secretive ruling party is uneasy about the public's eagerness to discuss politics and sensitive issues online despite threats of punishment.
In March, authorities scrambled to squelch online rumors about a possible coup amid a political crisis that led to the downfall of a prominent party figure, Bo Xilai, ahead of the party's fall leadership transition. A dozen websites were closed and six people detained.
This week, 70 prominent Chinese scholars and lawyers circulated an online petition this week appealing for free speech, independent courts and for the ruling party to encourage private enterprise.
Communist leaders who see the Internet as a promising source of economic growth were slow to enforce the same level of control they impose on movies, books and other media, apparently for fear of hurting e-commerce and other fledgling online businesses.
Until recently, Web surfers could post anonymous comments online or on microblogs.
That gave ordinary Chinese a unique opportunity to express themselves to a public audience in a society where newspapers, television and other media all are state-controlled. Some of the most popular microbloggers have millions of readers.
It also made the Internet a clearinghouse for accusations of official misconduct.
A local party official in China's southwest was fired in November after scenes from a videotape of him having sex with a young woman spread quickly on websites.
Web surfers can circumvent filters by using virtual private networks — encryption software that is used by companies for financial data and other sensitive information. But VPN users say disruptions began in 2011 and are increasing, suggesting regulators are trying to block encrypted traffic.
AP researcher Flora Ji contributed.